1. Field of the Invention
The present invention relates to load handling vehicles and more particularly to such vehicles having a rotatable grapple mechanism for handling loads, such as log and pulp wood loaders.
2. Description of the Prior Art
Prior art load handling vehicles having a rotatable grapple device are known. However, these vehicles typically have a rotary hydraulic or other motor which drives a chain or gear to rotate the grapple. Because of the high inertia associated with rotating heavy loads, such as logs, these grapple mechanisms are subject to failure, particularly when rotation of the load is halted. At such times, the gears are often stripped and drive chains broken when resisting the turning load. Although various mechanical braking devices have been employed to stop the rotating loads, these braking devices are also subject to failure because of the stresses placed on them when acting against the inertia of a heavy load.
Furthermore, precise positioning of the load with mechanical braking devices, gears, chain drives or other such mechanisms is extremely difficult.
To overcome the shortcomings of prior grapple rotating mechanisms, it would be desirable to use fluid-powered cylinders to achieve rotation of the grapple. Fluid power is readily available from many vehicles and fluid cylinders used in conjunction with levers can develop high turning and braking forces for their size and weight as compared to other fluid motors. Furthermore, precise positioning control is possible with fluid cylinders, and cylinder-operated mechanisms are usually rugged and easy to service. Also, they do not require the use of gears or a chain and sprocket to transmit or resist turning forces. However, in the known state of the art, fluid cylinder-operated rotation mechanisms are not used to rotate grapples, particularly grapples for handling heavy loads and grapples for which it is advantageous to be able to rotate through an arc of up to 180.degree. and also swing fore and aft.
Although some prior patents suggest cylinder-operated devices for rotating loads about a vertical axis, such devices either require a special purpose vehicle, or provide for rotation through only a limited arc, or would not be adapted to handle large, heavy loads such as logs. In addition, such prior devices are not suitable for operation within the space limitations of a grapple-type lift truck where the cylinder-operated rotation mechanism must be mounted between a pair of cantilevered boom portions and their grapple-swing cylinders. Examples of such prior cylinder-operated rotation mechanisms for special purpose vehicles, but unsuitable for use in a grapple-type lift truck, are found in U.S. Pat. Nos. 3,730,365 and 4,016,992.
Known prior grapple devices commonly use a pair of grapple-mounted cylinders to open and close the grapple arms. Such cylinders tend not to open and close the two grapple arms equally. Some prior grapple devices do use a single grapple-mounted cylinder and a mechanical linkage such as gears or chains to open and close the grapple arms together equally. However such prior linkages tend to be complex and subject to breakage, and hence not entirely satisfactory.
Thus, there is a need for a compact fluid cylinder-operated rotation mechanism capable of rotating a grapple through at least 180.degree. without interfering with the fore and aft powered movement of the grapple and capable of handling heavy loads. There is also a need for a rotatable grapple mechanism having a rugged, simplified powered mechanism for positive opening and closing of the grapple arms together equally.